In case anyone is having this issue independent of this plugin (such as after importing images directly through a custom script during a site transfer), here’s how I solved it:
INSERT INTO wp_postmeta
(post_id,meta_key,meta_value)
SELECT
p.ID post_id,
‘_wp_attached_file’ meta_key,
REPLACE(p.guid,’https://www.yoursite.com/wp-content/uploads/’,”) meta_value
FROM wp_posts p
LEFT OUTER JOIN wp_postmeta m
ON m.post_id=p.ID
WHERE p.post_type=’attachment’
AND p.post_parent!=0
AND (SELECT COUNT(e.post_id) FROM wp_postmeta e WHERE e.meta_key=’_wp_attached_file’ AND e.post_id = p.ID)=0
